The present invention relates to a hydraulic circuit system which is mounted on a construction machine including a plurality of hydraulic actuators often simultaneously operated, such as a hydraulic excavator, and which can provide a smooth start-up characteristic regardless of the magnitude of an inertia body to be driven.
There are two types of hydraulic circuit systems mounted on a construction machine such as a hydraulic excavator; one employing a center bypass control valve and including a bleed-off circuit, and the other employing a closed center control valve and including no bleed-off circuit. The latter hydraulic circuit system employs a load sensing system for controlling a delivery rate of a hydraulic pump so that a hydraulic fluid can be basically supplied at a flow rate demanded by the control valve. In the case of intending simplification of hydraulic equipment, the latter hydraulic circuit system is more advantageous because of including no bleed-off circuit. The absence of a bleed-off circuit however gives rise to the problem that, when a hydraulic actuator having large inertia is driven, the actuator is abruptly accelerated in a transient state due to a sudden rise of pressure, or the actuator is free from a smooth start-up characteristic because vibration of pressure (pressure pulsation) does not attenuate early.
More specifically, in the load sensing system, the delivery rate of the hydraulic pump is controlled so that the hydraulic fluid can be supplied at the flow rate demanded by the control valve. Accordingly, where a load to be driven by the actuator is an inertia body such as a swing body and the actuator cannot fully consume the hydraulic fluid delivered from the hydraulic pump, the delivery pressure of the hydraulic pump abruptly rises and the energy delivered from the hydraulic pump is accumulated in a piping system. Then, when the actuator has passed an acceleration range and pressure for acceleration is no longer required, the energy accumulated in the piping system is released upon lowering of the driving pressure, causing the actuator to overshoot. This overshoot further lowers the driving pressure. After that, the actuator speed is reduced, whereupon the driving pressure rises again, thus repeating changes in the actuator speed and the driving pressure. Stated otherwise, the actuator is brought into such a transient state that a sudden rise of pressure occurs and pressure pulsation does not attenuate early.
In view of the above problem, JP,A 4-191501, JP,A 5-263804, and JP,A 10-89304 propose methods for reducing a supply flow rate to the actuator with an increase of the driving pressure and suppressing a sudden rise of pressure.
The methods disclosed in JP,A 4-191501 and JP,A 5-263804 have the same purport and are intended to propose a control valve for controlling a displacement of a proportional seat valve having a slit in accordance with a valve opening of a pilot valve, wherein a displacement of the pilot valve is controlled depending on a driving pressure of an actuator to thereby control the displacement of the proportional seat valve. More specifically, a pressure having been introduced from an inlet portion of a hydraulic motor through a throttle is introduced to the pilot valve against the force acting upon the pilot valve for operation. The pressure having been introduced from the inlet portion of the hydraulic motor through the throttle is a pressure that increases in proportion to a driving pressure of the hydraulic motor. Therefore, the valve opening of the pilot valve is reduced in proportion to the driving pressure of the hydraulic motor, whereupon the valve opening of the proportional valve is also reduced. A hydraulic fluid delivered from a hydraulic pump is further controlled so as to reduce correspondingly. This reduction of the delivered hydraulic fluid contributes to moderating a sudden rise of pressure and attenuating pressure pulsation.
According to JP,A 10-89304, a pressure compensation valve provided for enabling the combined operation to be performed in the load sensing system is given with a load dependent characteristic that reduces a compensation differential pressure as a load pressure increases. This results in such control that as the load pressure increases, a supply flow rate to an actuator is reduced and a delivery rate of a hydraulic pump is also reduced. The load dependent characteristic of the pressure compensation valve is provided by setting, of pressure bearing areas of the pressure compensation valve, a pressure bearing area against which a pressure on the inlet side of a meter-in variable throttle acts in the closing direction, to be larger than a pressure bearing area against which a pressure on the outlet side of the meter-in variable throttle acts in the opening direction. By so setting a difference between both the pressure bearing areas, there occurs a hydraulic force that acts in the closing direction corresponding to the difference between both the pressure bearing areas, and is increased as the load pressure rises. In proportion to the load pressure, therefore, the differential pressure across the meter-in variable throttle is controlled so as to decrease and the supply flow rate to the actuator is reduced. With a reduction of the supply flow rate to the actuator, the delivery rate of the hydraulic pump under load sensing control is reduced. As a result, a sudden rise of pressure is avoided and pressure pulsation attenuates more early.
Meanwhile, JP,A 2-296002 proposes a hydraulic circuit system including a load sensing system, wherein a driving speed of a particular hydraulic actuator only is slowed down to achieve fine-speed operation without changing a target differential pressure of load sensing control set on pump control means. According to this proposal, a spring force of a check valve for detecting a load pressure is set to a certain degree of strength so that the load pressure is modulated with a pressure loss produced by the check valve. A detected signal pressure is lowered from the load pressure by an amount corresponding to the pressure loss, and a differential pressure between a delivery pressure of a hydraulic pump under the load sensing control and the load pressure is also lowered from an originally set value by an amount corresponding to the pressure loss. Consequently, the flow rate delivered under the load sensing control is reduced.
Further, PCT Laid-Open Publication WO98/31940 discloses a control valve for use in a hydraulic circuit system including a load sensing system, the control valve being constructed as a valve assembly in combination of a flow distribution valve and a hold check valve for simplification. In the disclosed control valve, a valve body of the flow distribution valve is partly incorporated in a hollow valve body of the hold check valve, a load pressure detecting hydraulic line of the control valve is formed as an internal passage (hydraulic line slit) of the flow distribution valve, and the internal passage is utilized to provide a check valve function. As a result, a check valve as a separate valve element is no longer required and the control valve is simplified in its overall construction.
With the proposals disclosed in JP,A 4-191501, JP,A 5-263804 and JP,A 10-89304, in proportion to the load pressure, the supply flow rate to the hydraulic actuator is reduced and the delivery rate of the hydraulic pump is also reduced. Upon driving of the hydraulic actuator, therefore, a sudden rise of pressure is avoided and pressure pulsation attenuates more early. A smooth start-up characteristic is thus obtained regardless of the magnitude of an inertia body to be driven. However, those prior-art techniques have the following problems.
The proposals disclosed in JP,A 4-191501 and JP,A 5-263804 are difficult to implement using an ordinary spool-type control valve from the structural point of view because the control valve employed in those proposals is constructed so as to control the valve opening of the proportional valve in accordance with the valve opening of the pilot valve. In a recent control valve, particularly, a spool inner space is utilized as a fluid passage for building a recovery circuit, and therefore a difficulty is doubled.
The proposal of JP,A 10-89304 discloses the valve structure of the pressure compensation valve adaptable for the case of using a spool-type control valve. Because the pressure compensation valve is constructed to have a certain difference between the pressure bearing areas, the structure is too complicated from the standpoint of assembly, and management of the pressure bearing areas is also troublesome.
The proposal of JP,A 2-296002 is intended to achieve fine-speed operation by slowing down the driving speed of the particular hydraulic actuator only. Despite such an intention, the delivery rate of the hydraulic pump is reduced, thus eventually resulting in that a sudden rise of pressure is avoided and pressure pulsation attenuates more early upon driving of the hydraulic actuator. Another advantage is that the structure is simplified because the pressure loss is just produced in the check valve for detecting the load pressure. However, the pressure loss produced in the check valve is set by the spring force and is a fixed value regardless of the load pressure. In other words, a control characteristic depending on the magnitude of an inertia body, i.e., a load dependent characteristic, is not obtained. This raises the problem that, depending on the magnitude of an inertia body to be driven, a sudden rise of pressure occurs and pressure pulsation does not attenuate early upon driving of the hydraulic actuator.
The control valve disclosed in PCT Laid-Open Publication WO98/31940 is constructed as a valve assembly in combination of a flow distribution valve and a hold check valve, and has various functions incorporated therein. The disclosed control valve is therefore advantageous in having a simplified overall construction. However, the disclosed control valve includes no measures against a sudden rise of pressure and pressure pulsation both occurred when an actuator having large inertia is driven. This raises the problem that, when a large inertia body is driven, a sudden rise of pressure occurs and pressure pulsation does not attenuate early upon driving of the hydraulic actuator.
An object of the present invention is to provide a hydraulic circuit system including a load sensing system, which can provide a smooth start-up characteristic regardless of the magnitude of an inertia body to be driven, and which has a simple construction and is easily adaptable even for a spool-type control valve.
(1) To achieve the above object, the present invention provides a hydraulic circuit system comprising a hydraulic pump, a plurality of hydraulic actuators driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of control valves disposed between the hydraulic pump and the plurality of actuators, a signal detecting hydraulic line to which a signal pressure based on a maximum load pressure among the plurality of hydraulic actuators is introduced, and pump control means for controlling a delivery pressure of the hydraulic pump to be held higher than the signal pressure by a predetermined value, the plurality of control valves comprising respectively main valves including meter-in variable throttles for controlling flow rates of the hydraulic fluid supplied to the hydraulic actuators, and flow distribution valves disposed between the meter-in variable throttles and the actuators, each of the flow distribution valves including a valve body which has one end positioned in an inlet passage connected to the meter-in variable throttle and the other end positioned in a control chamber, the valve body being moved through a stroke depending on balance between a pressure in the control chamber and a pressure in the inlet passage to control the pressure in the inlet passage, thereby controlling a differential pressure across the meter-in variable throttle, wherein the hydraulic circuit system further comprises a first hydraulic line provided in each of the plurality of control valves for, when a load pressure of the associated hydraulic actuator is the maximum load pressure, detecting that load pressure and introducing the detected load pressure to the control chamber; a second hydraulic line provided in each of the plurality of control valves for connecting the control chamber to the signal detecting hydraulic line and introducing the signal pressure in the signal detecting hydraulic line to the control chamber when the load pressure of the associated hydraulic actuator is not the maximum load pressure; a third hydraulic line for connecting the signal detecting hydraulic line to a reservoir; a first throttle disposed in the third hydraulic line; and a second throttle disposed in the second hydraulic line of at least one of the plurality of control valves for, when the load pressure of the associated hydraulic actuator is the maximum load pressure, cooperating with the first throttle to modulate that load pressure and introducing the modulated load pressure, as the signal pressure, to the signal detecting hydraulic line.
Since the first hydraulic line and the second hydraulic line are provided in each of the plurality of control valves and the second throttle for cooperating with the first throttle to modulate the load pressure introduced to the control chamber and introducing the modulated load pressure to the signal detecting hydraulic line is disposed in the second hydraulic line of at least one control valve, the differential pressure across the second throttle is increased as the load pressure (maximum load pressure) of the hydraulic actuator associated with the at least one control valve rises, and the action of reducing the signal pressure introduced to the signal detecting hydraulic line is enhanced. Since the pump control means controls the delivery pressure of the hydraulic pump to be held higher than the signal pressure by the predetermined value, the differential pressure across the meter-in variable throttle of the relevant control valve is reduced as the load pressure rises, whereby the action of reducing a controlled flow rate is developed. At the start-up of the hydraulic actuator associated with the particular control valve, therefore, a supply flow rate to the associated hydraulic actuator is reduced depending on the load pressure, and a delivery rate of the hydraulic pump is also reduced. Accordingly, a sudden rise of pressure is avoided and hydraulic pressure pulsation attenuates more early upon driving of the hydraulic actuator. A smooth start-up characteristic is thus obtained regardless of the magnitude of an inertia body to be driven.
Further, since the second throttle is just additionally disposed in the second hydraulic line, the construction is very simple and easily adaptable even for a control valve having a main valve of the spool type. Also, there is no risk of a malfunction because the second throttle is just added.
(2) In above (1), preferably, the plurality of control valves further comprise respectively hold check valves disposed between the flow distribution valves and the hydraulic actuators whereby the first hydraulic lines detect, as the load pressures, pressures between the meter-in variable throttles and the hold check valves.
With those features, even when the load pressure of the hydraulic actuator becomes higher than the pressure at the meter-in throttle of the main valve, the load pressure is held by the hold check valve and the hydraulic fluid is prevented from flowing backward to the reservoir through the first hydraulic line, the second hydraulic line, the second throttle, the signal detecting hydraulic line, the third hydraulic line and the first throttle.
(3) In above (1) or (2), preferably, the flow distribution valve includes a hydraulic line slit formed in an outer periphery of the valve body thereof and opened to an outlet passage of the flow distribution valve, and a lap portion provided between the hydraulic line slit and the control chamber for making the hydraulic line slit open to the control chamber when the valve body of the flow distribution valve is moved through a stroke of predetermined distance in the valve opening direction, the hydraulic line slit and the lap portion jointly forming the first hydraulic line.
With those features, the first hydraulic line of the control valve is constituted as an internal passage (hydraulic line slit) of the flow distribution valve, and the check valve function is provided by utilizing the internal passage (hydraulic line slit). Therefore, the overall construction of the control valve is simplified.
(4) In above (1) or (2), preferably, the valve body of each flow distribution valve of the plurality of control valves has a pressure bearing area on the side of the inlet passage larger than a pressure bearing area on the side of the control chamber.
With that feature, characteristics of the control valve on the lower load pressure side is also improved in, for example, removing the influence of a flow force acting upon the flow distribution valve of the control valve on the lower load pressure side during the combined operation, and therefore better combined operation is achieved. Further, means, described in above (1), for improving characteristics of the control valve on the higher load pressure side and means (for changing the pressure bearing area) for improving the characteristics of the control valve on the lower load pressure side are independent of each other. Therefore, an improvement in characteristic of the control valve on the higher load pressure side and an improvement in characteristics of the control valve on the lower load pressure side can be achieved by mutually independent means, and flexibility in selection of equipment is increased to a large extent.
(5) In above (1) or (2), preferably, the second throttle is a variable throttle, and means for adjusting an opening area of the variable throttle is provided.
With those features, the opening area of the second throttle is freely adjustable and an optimum load dependent characteristic can be set depending on the type of actuator load.
(6) To achieve the above object, the present invention also provides a hydraulic circuit system comprising a hydraulic pump, a plurality of hydraulic actuators driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of control valves disposed between the hydraulic pump and the plurality of actuators, a signal detecting hydraulic line to which a signal pressure based on a maximum load pressure among the plurality of hydraulic actuators is introduced, and pump control means for controlling a delivery pressure of the hydraulic pump to be held higher than the signal pressure by a predetermined value, the plurality of control valves comprising respectively main valves including meter-in variable throttles for controlling flow rates of the hydraulic fluid supplied to the hydraulic actuators, and pressure compensation valves disposed between the hydraulic pump and the meter-in variable throttles for controlling differential pressures across said meter-in variable throttles, wherein the hydraulic circuit system further comprises first hydraulic lines provided respectively in the plurality of control valves for introducing load pressures of the associated hydraulic actuators to pressure bearing sectors of the pressure compensation valves and controlling the differential pressures across the meter-in variable throttles; second hydraulic lines provided respectively in the plurality of control valves for detecting the load pressures of the associated hydraulic actuator; selecting means for detecting a maximum one of pressures in the second hydraulic lines of the plurality of control valves and introducing the detected maximum pressure, as the signal pressure, to the signal detecting hydraulic line; a third hydraulic line for connecting the signal detecting hydraulic line to a reservoir; a first throttle disposed in the third hydraulic line; and a second throttle disposed in the second hydraulic line of at least one of the plurality of control valves for, when the load pressure of the associated hydraulic actuator is the maximum load pressure, cooperating with the first throttle to modulate that load pressure and introducing the modulated load pressure, as the signal pressure, to the signal detecting hydraulic line.
With those features, the similar working advantages to those described in above (1) can be obtained in a hydraulic circuit system including a before-located-type flow distribution valve (pressure compensation valve).